The present invention relates to a fluid pumping device, and additionally, to a high-efficiency, axial flow marine propulsion system.
In a multistage axial flow pump, i.e. two or more stages, energy is transferred from a prime mover or engine to the working fluid (e.g., water) at each stage. Pressure is increased at each succeeding stage until the working fluid is exhausted through a discharge nozzle to generate thrust. Increased pressure inside the pump tends to suppress damaging cavitation that may otherwise act upon the impellers. This differs from a conventional centrifugal or mixed-flow pumping device that is generally limited to single stage and consumes a relatively large volume. Apart from marine propulsion, other large-scale pumping applications of the invention include fire control, flood control, irrigation, and in large cooling towers.
An axial-flow pumping device used in marine propulsion, for example, might include an outer casing or housing, a water inlet, a shaft-driven impeller section, and an outlet or discharge nozzle. Such devices were either single stage or provided counter-rotating rotors in two stages. Counter-rotating rotors, however, presented myriad mechanical problems and were difficult to service. The impeller section included multiple radially aligned rotor blades mounted on a rotating wheel or rotor that forced water from the inlet to the outlet. Power was derived from a conventional power plant, such a piston-driven gasoline or diesel engine, a gas or steam turbine engine, or any combination thereof. A drive shaft and sometimes, a gear reduction mechanism, coupled the prime mover to the impeller section of the pumping device to convert rotary power to thrust.
Most engines, however, have only one optimum operating speed that delivers peak horsepower or peak efficiency, but the operating speed may not optimally match the desired thrust and/or hull speed of the vessel, which varies with loading of the vessel, fluid density, fluid temperature, or other conditions. A fully loaded vessel, for example, has a different optimum operating speed than a lightly loaded vessel. Thus, certain inefficiencies inherently exist in prior power plant-thruster combinations.
To compensate for inefficiencies, prior axial flow devices employed variable pitch rotor blades in the impeller section to match the optimum torque, speed, or fuel efficiency of the prime mover. It is known in a prior pumping device, but not necessarily applied to marine propulsion, to include fixed stator vanes between impeller sections of a multi-stage pumping device to counteract whirl or rotational velocity that the rotor blades impart to the fluid, such as that disclosed by U.S. Pat. Nos. 5,755,554 and 5,562,405 (both issued to Ryall). The stator vanes had the effect of redirecting fluid flow to maintain a desired angle-of-attack of rotor blades in the succeeding stage while the rotor blades worked against the whirling fluid, but such prior stator vane designs significantly increased internal friction. It was not known, however, to provide variable pitch stator vanes in prior systems to efficiently compensate for pressure, velocity (propulsor or vessel), or torque fluctuations. Ryall, for example, provides a substantially constant absolute velocity in flow passages between fixed stator blades. Due to their geometric structure, prior stator vane designs did not maintain or increase static pressure between rotor sections, and therefore, endured other losses in efficiencies. Such prior systems generally operated, at best, around 65 to 72% propulsive efficiency.
The pumping or propulsion device of the present invention, however, uses multiple rotor-stator stages that include geometrically efficient blades and vanes, e.g., an airfoil shape, to minimize internal drag and to successively increase static pressure of the working fluid at each stage of the device. Because fluid velocity decreases across the stator vanes, static pressure increases thereby improving overall efficiency of the device. Variable-pitch stator vanes may also be employed to further improve efficiency since pitch angle changes altered the angle of attack of working fluid against rotor blades in the succeeding section. Varying the angle of attack impacted the torque required by the prime mover to drive the pump.
Preferably, the rotor blades in each section of a preferred multi-stage pump or propulsion device are fixed-pitched thereby obviating mechanical problems typically associated with variable-pitch rotor blades. Thin, low-drag stator vanes, fixed or variable-pitch, are also preferred to minimize internal drag. Advantageously, the improved multistage structure has a simpler mechanical construction, has a larger thrust-weight ratio, is more easily serviced and maintained, and importantly, achieves greater propulsive efficiencies, i.e., in the range of 84 to 90% (or more), regardless of the thrust and/or hull speed set points. When deployed in marine propulsion, the present invention may additionally include a variable area discharge nozzle, i.e., a controllable throat area, to optimally match vessel speed with the discharge speed of the water jet for any given or desired thrust or power setting. This enables the vessel to operate at maximum propulsive efficiency over a wide range of speeds thereby conserving precious fuel and increasing range.